In article JU60j.8639$fD.1416@pd7urf3no,
Tony Hwang wrote:

Hi,
What is toughest drill bit for drilling many, many holes
thru a bolt stuck in a confined space. I am trying to drill
it into bits to break it off. There is no other means to get rid of it.
Cobalt bit or any other better one?
TIA,

Toughest standard bit is solid carbide. But, probably too brittle for
use in a hand held drill. Carbide tipped might be OK, or cobalt as you
said. Your ability to do this successfully is going to be dependent on
several factors, including the hardness of the bolt in question, and
your skill. Speeds and feeds are going to be important. Step drilling is
essential.

Do use sharp drills.
Do use a goodly amount of pressure.
Do use a fairly low RPM.
Do use cutting oil.

If you screw this up you'll end up with drill bits broken in holes, or
work-hardened steel. Either will make a difficult job more so.